Conventionally, as a binder for foundry sand for the production of molds, an acid curing resin, such as a phenolic resin, a furan resin, and furfuryl alcohol, is used, and a method of producing a mold wherein these binders are cured by adding a curing agent thereto is usually carried out. A furan resin having good properties as an organic self-curing resin for foundry sand has long been used (Japanese Patent Publication No. 39-1543/1964, etc.), and such a furan resin has been improved in various ways to develop, depending on its application, for example, a binder which is weaken in odor by adding thereto glyoxal, tetraoxane or the like and a binder is lowered in cost by modifying it with a phenolic resin, a urea resin or the like.
However, there has recently been a demand for an improvement in working efficiency through improvements in machinery, facilities, processes, and the like for the production of castings and in association with it, as a binder for foundry sand, a self-curing organic binder of a quick curing type has been strongly desired. To meet the demand, for example, a method wherein the sand temperature is elevated in order to accelerate the curing of a binder, a method wherein water is removed under reduced pressure, and a method wherein use is made of a large quantity of a curing agent have been adopted.
However, for such a purpose, to elevate the temperature is economically disadvantageous because extra energy is required. Further to remove water under reduced pressure can accelerate the curing to a certain extent but cannot fundamentally solve the problem. Further the use of a large quantity of a curing agent can improve the curing speed to accelerate the curing, but results in such disadvantages that the working atmosphere is deteriorated by the decomposed gases and that the strength of castings was lowered.
In view of the above, as a curing agent composition that makes the working atmosphere difficultly deteriorated even if the curing agent composition is used in a relatively large quantity, one wherein a phosphoric acid compound and a sulfonic acid compound are mixed in a specified ratio is suggested (Japanese Patent Application Laid-Open No. 5-237587). That is, by mixing a phosphoric acid compound low in toxicity with a sulfonic acid compound capable of promoting curing in a specified ratio, it is intended to accelerate the curing speed and to prevent the working atmosphere from being deteriorated. Although such a curing agent composition was very useful, it is needless to say that if the usage was too much, the working atmosphere was deteriorated disadvantageously. Further the above official gazette does not describe at all the use of a furan resin together with a curing accelerator and the adjustment of the degree of polycondensation of that furan resin.